Phenolic resin composition for shell molding, resin coated sand for shell molding, and shell mold formed of the same

ABSTRACT

Provided are a phenolic resin composition for shell molding that has low thermal expansion properties and high flexibility, a resin coated sand for shell molding obtained by using the same, and a shell mold formed of the same. The phenolic resin composition for shell molding that is capable of exhibiting advantageous mold characteristic is obtained by a combination of a phenolic resin that is obtained by a reaction of a phenol, a naphthol, and an aldehyde, and a fatty acid amide.

This application is a continuation of the International Application No. PCT/JP2010/061591 filed on Jul. 8, 2010, which claims the benefit under 35 U.S.C. §119(a)-(d) of Japanese Patent Application 2009-172135, filed on Jul. 23, 2009, the entireties of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a phenolic resin composition for shell molding, a resin coated sand for shell molding, and a shell mold formed of the same. In particular, the present invention relates to a phenolic resin composition for shell molding that simultaneously solves problems related to thermal expansion and flexibility, a resin coated sand obtained by using the phenolic resin composition, and a shell mold obtained by using the resin coated sand.

BACKGROUND ART

Conventionally, in shell-mold casting, there is generally used a shell mold that is formed by hot-forming a resin coated sand obtained by kneading a fire-refractory particle (casting sand) and a phenolic resin (binder), and as necessary a hardener such as hexamethylenetetramine, into a desired shape. Hereinafter, the resin coated sand is referred to as “RCS”.

However, in casting process by using this kind of mold, especially by using a mold which has a complex shape, e.g., a mold for casting a molded product such as a cylinder head of an internal combustion engine, there is a problem that a fracture or a crack (hereinafter referred to as “crack” of the mold) is easily caused on the mold during the casting process using the mold.

Meanwhile, it is conceivable that the crack of a mold can be prevented by lowering coefficient of thermal expansion and increasing the flexibility of mold. Patent document 1 discloses that coefficient of rapid thermal expansion is lowered by using bisphenol such as bisphenol A and bisphenol E as a component of binder, so that low thermal expansion properties are obtained. However, although such technique has sufficiently solved the problem of crack of the mold, the technique has not sufficiently solved the problem of flexibility.

Patent document 2 proposes a method in which crack of the mold is prevented by incorporating polyethylene glycol having a number average molecular weight of 1500 to 40000 into RCS. However, thermal expansion properties and flexibility are not sufficiently improved by this method, and thus further improvement is needed.

Patent document 3 discloses that by using RCS formed by coating surface of a casting sand with a phenolic resin excellent in collapse resistance, which is produced by using at least naphthol as phenol component, the improvement of regeneration rate of the used shell sand and the stability of quality of the regenerated sand can be obtained because collection of a mass of shell when the mold is broken down after molding is improved. In examples of patent document 3, a phenolic novolak resin and a phenolic resole resin are exemplified that are obtained by a reaction of α-naphthol, β-naphthol, or a combination of these naphthols, a phenol, and a formalin in the presence of a catalyst such as hydrochloric acid and ammonia water. However, particularly in the production of such resin by using the hydrochloric acid as a catalyst, there is a safety problem caused by vigorous reaction during the production of the resin, and also there is a problem of corrosion of a die during the production of the mold. Further, patent document 3 is silent about a phenolic resin obtained by using an oxalic acid as a catalyst and RCS obtained by using the same. Furthermore, it is also silent about a crack of a mold which should be considered when producing a mold.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-59-178150 -   Patent Document 2: JP-A-58-119433 -   Patent Document 3: JP-A-63-30144

SUMMARY OF INVENTION

The present invention has been made in the light of the situations described above. It is therefore an object of the present invention to provide: a phenolic resin composition for shell molding that has low thermal expansion properties and high flexibility; RCS obtained by using the phenolic resin: a process for producing the RCS; and a shell mold obtained by using such RCS.

The inventors of the present invention have conducted intensive study and research about the phenolic resin composition for shell molding in an effort to solve the above-described problems and found that a phenolic resin composition having effective properties can be obtained by a reaction of phenol components including a phenol and a naphthol with an aldehyde in the presence of a reaction catalyst such as a divalent metal salt and/or an oxalic acid. Specifically, they found that in the mold produced by using RCS formed by using the above-described phenolic resin composition, low coefficient of thermal expansion and high flexibility are obtained. Thus, the present invention has been completed.

It is therefore a gist of the present invention to provide a phenolic resin composition for shell molding, comprising as essential components: a phenolic resin obtained by a reaction of a phenol, a naphthol, and an aldehyde; and a fatty acid amide.

According to a preferable aspect of the phenolic resin composition for shell molding of the present invention, a ratio of the phenol to the naphthol is in a range of from 95:5 to 50:50 by mass ratio.

According to another preferable aspect of the present invention, the naphthol comprises 1-naphthol and/or 2-naphthol.

According to a further preferable aspect of the present invention, a reaction molar ratio among the phenol (P), the naphthol (N), and the aldehyde (F): F/(P+N) is in a range of from 0.40 to 0.80.

According to a preferable aspect of the present invention, the fatty acid amide is present in a range of from 1 to 15 parts by mass based on 100 parts by mass of the phenolic resin.

According to a favorable aspect of the present invention, the fatty acid amide is one of a monoamide, a substituted amide, and a bisamide.

According to a still further preferable aspect of the present invention, the fatty acid amide is a fatty acid bisamide, more preferably, a saturated fatty acid bisamide.

According to another favorable aspect of the present invention, the phenolic resin composition further comprises a silane coupling agent.

It is another gist of the present invention to provide RCS (resin coated sand) for shell molding characterized in that a fire-refractory particle is coated with the phenolic resin composition for shell molding according to the above aspects.

According to a preferable aspect of the RCS for shell molding of the present invention, the phenolic resin composition is present in a range of from 0.2 to 10 parts by mass based on 100 parts by mass of the fire-refractory particle.

It is a still further gist of the present invention to provide a shell mold obtained by forming and heat-curing the resin coated sand for shell molding according to the above aspects.

It is still further gist of the present invention to provide a process for producing a resin coated sand, comprising the steps of: (a) reacting a phenol, a naphthol, and an aldehyde in the presence of a catalyst to obtain a phenolic resin; and (b) coating a fire-refractory particle with the phenolic resin and a fatty acid amide, which are mixed by melting, or coating a fire refractory particle with the phenolic resin and a fatty acid amide, which are used independently.

According to a preferable aspect of the present invention, the catalyst comprises a divalent metal salt and/or an oxalic acid.

The phenolic resin composition for shell molding according to the present invention includes a phenolic resin that is obtained by a reaction of a phenol, a naphthol, and an aldehyde, and a fatty acid amide. Therefore, when a coating layer including the phenolic resin composition is formed on a surface of a predetermined fire-refractory particle so as to constitute RCS for shell molding and such RCS is used to produce a shell mold, the obtained mold has low thermal expansion properties and the flexibility of the mold can be sufficiently improved. Accordingly, a problem of casting defect of veining caused by a crack of the mold can be solved at the same time. In addition, since the phenolic resin can be produced without a corrosive component such as hydrochloric acid, a problem of corrosion of a die during mold-forming may not be caused. Thus, the present invention can have industrial advantages that the intended shell mold can be easily and safely produced.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is an explanatory view showing how the “flexibility” of mold is measured in examples.

DETAILED DESCRIPTION OF THE INVENTION

The phenolic resin constituting the phenolic resin composition for shell molding of the present invention is obtained by a reaction of a phenol, a naphthol, and an aldehyde in the presence of a predetermined catalyst.

Here, examples of the phenol which is one of reaction components of a phenolic resin include conventionally known phenol, for example, phenol, alkylphenols such as cresol, xylenol, p-tert-butylphenol and nonylphenol, polyhydric phenols such as resorcinol, bisphenol F and bisphenol A, and a mixture thereof. Any one of, or any combination thereof may be used.

The present invention is characterized by that the naphthol is used as a phenol component together with the phenol. Due to this characteristic, the properties of the phenolic resin to be obtained are effectively improved. In terms of its ready availability and a reduction of cost, for example, 1-naphthol, 2-naphthol, and a mixture thereof may be used as the naphthol. Preferably, 1-naphthol is employed because of its excellent reactivity with aldehyde, for example. The phenol and naphthol are employed such that the ratio of phenol to naphthol (1-naphthol and/or 2-naphthol) is in a range of from 95:5 to 50:50 by mass. In other words, the naphthol is employed so as to be present in an amount of 50% by mass or less, based on the total phenol component. When the amount of the naphthol is more than 50% by mass, an amount of generation of tar during casting may be increased. On the other hand, when the amount of the naphthol is less than 5% by mass, flexibility may not be sufficiently exhibited. The ratio of phenol to naphthol is preferably in a range of from 90:10 to 60:40, more preferably from 90:10 to 70:30, in view of the strength of the mold.

Examples of the aldehyde, which is reacted with the above described phenol and naphthol to obtain the phenolic resin of the present invention, include formalin, paraformaldehyde, trioxan, acetaldehyde, paraldehyde, and propionaldehyde. It is to be understood that the aldehyde is not limited to the above examples, and other well-known materials may be suitably used. Any one of, or any combination of the aldehyde may be used.

In the present invention, in order to obtain the intended phenolic resin by reacting the phenol (P) and the naphthol (N) with the above-described aldehyde (F), it is recommended that the phenol and the naphthol are reacted with the aldehyde such that the blending molar ratio: F/(P+N) is in a range of 0.40 to 0.80. By controlling the blending molar ratio: F/(P+N) so as to be 0.75 or less, more preferably 0.70 or less, the flexibility can be further improved. In addition, by controlling the value of F/(P+N) so as to be 0.40 or more, the intended phenolic resin can be produced with a sufficient yield, and by controlling the value of F/(P+N) so as to be 0.80 or less, the strength of the mold which is obtained by using RCS for shell molding produced by using thus obtained phenolic resin can be advantageously improved.

In the present invention, any conventionally known catalyst such as an acid catalyst is suitably used in the reaction of the phenol and the naphthol with the aldehyde. Especially, it is recommended that at least one of a divalent metal salt and an oxalic acid be used as the catalyst. By using such a specific catalyst, coefficient of thermal expansion and flexibility can be further improved, and problems of metal corrosion and the like can be advantageously solved. Examples of the divalent metal salt include lead naphthenate, zinc naphthenate, lead acetate, zinc acetate, zinc borate, lead oxide, and zinc oxide, which are metal salts having divalent metal element, and a combination of an acidic catalyst, which is capable of forming the metal salt, and a basic catalyst. Among the specific catalysts, oxalic acid is preferably used. Generally, the catalyst including at least one selected from a group consisting of the divalent metal salts and the oxalic acid is present in an amount of 0.01 to 5 parts by mass, preferably 0.05 to 3 parts by mass, based on 100 parts by mass of the total of phenol and naphthol.

The reaction of phenol, naphthol, and aldehyde in the presence of the above-described catalyst is conducted in the same manner as a conventional production method of phenolic resin. Thus obtained phenolic resin is in a solid or a liquid (for example, varnish or emulsion) form, and expresses a heat-curing or -hardening effect when it is heated in the presence or absence of a hardener or curing catalyst such as hexamethylene tetramine. In the present invention, a phenolic resin having a number average molecular weight as measured by gel permeation chromatography (GPC) in a range of from 400 to 1300 is preferably used. When the number average molecular weight of the phenolic resin is too small, a mold to be obtained may not have sufficient strength, because RCS for shell molding which is coated with the resin composition including the phenolic resin has poor filling properties in mold-forming. On the other hand, when the number average molecular weight of the phenolic resin is too big, a mold to be obtained may not have sufficient strength, because flowability of resin during heating is deteriorated.

In the present invention, the fatty acid amide is added as an essential component to the above phenolic resin to obtain the phenolic resin composition for shell molding. Due to the combination of the phenolic resin and the fatty acid amide, low thermal expansion properties and improved flexibility can be advantageously achieved. The ratio of the phenolic resin to the fatty acid amid is suitably determined depending on required properties for a mold to be obtained. Generally, 1 to 15 parts by mass of the fatty acid amide is added based on 100 parts by mass of the phenolic resin. This is because, when the amount of the fatty acid amide is too small, advantages and effects to be obtained by using the fatty acid amide may not be sufficiently exhibited. On the other hand, when the amount of the fatty acid amide is too big, the advantages and effects that are of equal worth to the amount of the fatty acid amide may not be obtained.

Examples of the fatty acid amide, which is used in combination with the phenolic resin, include: monoamides such as saturated fatty acid monoamide and unsaturated fatty acid monoamide; substituted amides; and bisamides such as saturated fatty acid bisamide, unsaturated fatty acid bisamide, and aromatic bisamide. Of those fatty acid amides, the fatty acid bisamide, especially, the saturated fatty acid bisamide is favorably used.

Of the above fatty acid amide, examples of the saturated fatty acid monoamide include lauramide, myristamide, palmitamide, stearamide, and behenamide. Examples of the unsaturated fatty acid monoamide include oleamide, and erucamide. Examples of the substituted amides include N-stearyl(stearamide), N-oleyl(stearamide), N-stearyl(erucamide), methylol(stearamide), and methylol(behenamide). In addition, examples of the saturated fatty acid bisamide include methylenebis(stearamide), ethylenebis(stearamide), methylenebis(lauramide), methylenebis(behenamide), hexamethylenebis(stearamide), hexamethylenebishydroxyl(stearamide), N,N′-distearyl(adipamide) and ethylenebis(behenamide). Examples of the unsaturated fatty acid bisamide include ethylenebis(oleamide), ethylenbis(erucamide), hexamethylenbis(oleamide), and N,N′-dioleyl(adipamide). Further, examples of the aromatic bisamide include xylylenebis(stearamide), xylylenebishydroxy(stearamide), and N,N′-distearyl(isophthalamide).

In the present invention, in order to use the phenolic resin and the fatty acid amide in combination for shell molding, various conventionally known additives can be added for the purpose of improving the physical characteristics of the mold, for example. Examples of the additives include silane coupling agent such as γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane. Generally, such a silane coupling agent is added in a range of from about 0.01 to about 5 parts by mass, preferably 0.05 to 2.5 parts by mass, based on 100 parts by mass of the phenolic resin.

In the production of RCS for shell molding according to the present invention, the above-described phenolic resin composition for shell molding are kneaded into a fire-refractory particle. Because an amount of the phenolic resin composition for shell molding in RCS of the present invention is determined depending on a kind of resin to be used and strength of the intended mold, for example, the amount thereof is not necessarily limited. However, the phenolic resin composition is generally present in a range of from about 0.2 to about 10 parts by mass, preferably 0.5 to 8 parts by mass, more preferably 0.5 to 5 parts by mass, based on 100 parts by mass of the fire-refractory particle.

In the present invention, the kind of fire-refractory particle kneaded into the phenolic resin composition for shell molding is not particularly limited. As the fire-refractory particle is a basic material for a mold, any known inorganic particles conventionally used in the shell mold casting may be used as long as they have fire resistance suitable for casting and particle diameter suitable for forming a mold (mold-forming). In addition to a silica sand which is commonly used, examples of the fire-refractory particle include, special sands such as an olivine sand, a zircon sand, a chromite sand and an alumina sand, slag particles such as a ferrochromium slag, a ferronickel slag and a converter slag, mullite-based sand particles such as Naigai Cerabeads (commercial name, available from ITOCHU CERATECH CORP., JAPAN), and regenerated particles which are obtainable by recovering and regenerating the above particles after casting. Any one of, or any combination of the particles may be used.

In the production of RCS for shell molding, examples of the production method include, but are not limited to, any conventional methods such as a dry-hot-coating, a semi-hot-coating, a cold coating, and a powder-solvent-coating. In the present invention, a so-called dry-hot-coating is preferably recommended that includes the steps of kneading a preheated fire-refractory particle and a resin composition for shell molding in a mixer such as a whirl mixer or a speed mixer, adding aqueous hexamethylenetetramine (hardener) solution, converting an aggregated content into particles by being collapsed by cooling with an air blow, and adding a calcium stearate (lubricant). The predetermined phenolic resin and fatty acid amide included in the resin composition for shell molding of the present invention can not only be melted and mixed with each other to coat the fire-refractory particle, but also can independently be used to coat the fire-refractory particle.

Further, when making a predetermined shell mold by using the above-described RCS for shell molding, the process for making or forming a mold by heating is not particularly limited. Any known process may be advantageously employed. For example, a casting mold can be obtained by the steps of: filling a forming die, which has a cavity corresponding to a shape of an intended shell mold and is heated to 150 to 300° C., with the above-described RCS by a gravity-driven method or blowing method; curing the RCS; and removing the cured (hardened) mold from the forming die. The mold obtained as above advantageously has the above-mentioned excellent effect.

EXAMPLES

To further clarify the present invention, some examples of the invention will be described. It is to be understood that the invention is not limited to the details of the illustrated examples and the foregoing description, but may be embodied with various changes, modifications and improvements, which may occur to those skilled in the art without departing from the scope of the invention.

Here, “parts” and “%” in the following description refer to “parts by mass” and “% by mass”, respectively, unless otherwise defined. In addition, characteristics of the produced RCS for shell molding are measured in accordance with the following test methods.

—Evaluation of Flexibility of Mold—

Initially, for the evaluation of flexibility of mold, a piece of mold (120 mm×40 mm×5 mm) made of each kind of RCS was prepared under a cure condition: at 250° C. for 40 seconds. Then, the piece of mold was left until it was cooled to a room temperature.

Subsequently, thus obtained piece of mold was set on a support as shown in FIG. 1, and an exothermic stick (Erema exothermic stick) was gradually heated from 200° C. to 800° C. Meanwhile, a laser displacement gauge was positioned 10 mm away from an end portion of the piece of mold, and data thereof was directly entered into a computer. Behaviors with respect to the displacement were as follows: at first the piece of mold was warped due to an expansion behavior caused by the heating of the piece of mold; then the piece was started to be bent; and finally, the piece of mold was fractured almost at the center thereof, i.e., at the position heated by the exothermic stick. The term “flexibility” used herein is expressed by the maximum deflection obtained before the piece of mold was fractured. The higher value of the flexibility indicates that the mold is more easily deformed, which means that the mold is flexible. This measurement was conducted in consideration of measurement cycle such that the next measurement of a piece of mold starts when the temperature of the exothermic stick is reached around 200° C.

—Evaluation of Coefficient of Thermal Expansion—

Evaluation of coefficient of thermal expansion was conducted in accordance with a test method of rapid coefficient of thermal expansion specified in JACT test method M-2, test method of coefficient of thermal expansion. A test piece (diameter of 28.3 mm×length of 51 mm, cut into about ¼ of circumference) produced by heating a piece of mold at a temperature of 280° C. for 120 seconds was placed in a high-temperature casting sand tester controlled at 1000° C. and was taken out from it after 1 minute. A coefficient of thermal expansion was calculated by the lengths of the test piece before the heating and after the heating according to the following formula.

A coefficient of thermal expansion(%)={length of test piece(after heating−before heating)×100}/(length of test piece before heating)

Resin Production Example 1

To a reaction vessel provided with a thermometer, a stirring device, and a condenser, 8000 parts of phenol, 2000 parts of 1-naphthol, 4106 parts of 47% formalin, and 30 parts of oxalic acid were charged. Subsequently, a temperature in the reaction vessel was gradually raised to a reflux temperature and the mixture was subjected to a reaction under reflux condition for 90 minutes. Further, the mixture was dehydrated under ordinary pressure and heated under reduced pressure until the temperature reached 180° C. Accordingly, unreacted phenol was removed and phenolic resin A was obtained.

Resin Production Example 2

Phenolic resin B was obtained in the same way as Resin Production Example 1 with the exception that 8000 parts of phenol, 2000 parts of 1-naphthol, 4865 parts of 47% formalin and 30 parts of oxalic acid were charged.

Resin Production Example 3

Phenolic resin C was obtained in the same way as Resin Production Example 1 with the exception that 8000 parts of phenol, 2000 parts of 1-naphthol, 3159 parts of 47% formalin and 30 parts of oxalic acid were charged.

Resin Production Example 4

Phenolic resin D was obtained in the same way as Resin Production Example 1 with the exception that 9000 parts of phenol, 1000 parts of 1-naphthol, 4260 parts of 47% formalin and 30 parts of oxalic acid were charged.

Resin Production Example 5

Phenolic resin E was obtained in the same way as Resin Production Example 1 with the exception that 6000 parts of phenol, 4000 parts of 1-naphthol, 3799 parts of 47% formalin and 15 parts of oxalic acid were charged.

Resin Production Example 6

Phenolic resin F was obtained in the same way as Resin Production Example 1 with the exception that 8000 parts of phenol, 2000 parts of 2-naphthol, 4106 parts of 47% formalin and 30 parts of oxalic acid were charged.

Resin Production Example 7

Phenolic resin G was obtained in the same way as Resin Production Example 1 with the exception that 2000 parts of phenol, 8000 parts of bisphenol A (BPA), 2339 parts of 47% formalin and 30 parts of oxalic acid were charged.

Example 1

50 parts of ethylenebis(stearamide) and 10 parts of silane coupling agent (3-aminopropyltriethoxysilane) were mixed into 1000 parts of phenolic resin A by heating and melting to obtain Resin composition 1.

Example 2

Resin composition 2 was obtained in the same way as Example 1 with the exception that the amount of ethylenebis(stearamide) was changed to 120 parts.

Example 3

Resin composition 3 was obtained in the same way as Example 1 with the exception that the amount of ethylenebis(stearamide) was changed to 15 parts.

Example 4

Resin composition 4 was obtained in the same way as Example 1 with the exception that methylenebis(stearamide) was used instead of ethylenebis(stearamide).

Example 5

Resin composition 5 was obtained in the same way as Example 1 with the exception that ethylenebis(behenamide) was used instead of ethylenebis(stearamide).

Example 6

Resin composition 6 was obtained in the same way as Example 1 with the exception that ethylenebis(erucamide) was used instead of ethylenebis(stearamide)

Example 7

Resin composition 7 was obtained in the same way as Example 1 with the exception that stearamide was used instead of ethylenebis(stearamide).

Example 8

Resin composition 8 was obtained in the same way as Example 1 with the exception that phenolic resin B was used instead of phenolic resin A.

Example 9

Resin composition 9 was obtained in the same way as Example 1 with the exception that phenolic resin C was used instead of phenolic resin A.

Example 10

Resin composition 10 was obtained in the same way as Example 1 with the exception that phenolic resin D was used instead of phenolic resin A.

Example 11

Resin composition 11 was obtained in the same way as Example 1 with the exception that phenolic resin E was used instead of phenolic resin A.

Example 12

Resin composition 12 was obtained in the same way as Example 1 with the exception that phenolic resin F was used instead of phenolic resin A.

Comparative Example 1

Resin composition 13 was obtained in the same way as Example 1 with the exception that phenolic resin G was used instead of phenolic resin A.

Comparative Example 2

Resin composition 14 was obtained in the same way as Example 1 with the exception that no fatty acid amide was added to the phenolic resin A.

Comparative Example 3

Resin composition 15 was obtained in the same way as Example 1 with the exception that no fatty acid amide was added to the phenolic resin D.

Comparative Example 4

Resin composition 16 was obtained in the same way as Example 1 with the exception that no fatty acid amide was added to the phenolic resin E.

Comparative Example 5

Resin composition 17 was obtained in the same way as Example 1 with the exception that no fatty acid amide was added to the phenolic resin F.

Production Example 1 of RCS

To a laboratory whirl mixer, 7000 parts of fire-refractory particle (regenerated silica sand) heated to 130 to 140° C. and 105 parts of the phenolic resin composition obtained in each of the above Examples 1 to 12 and Comparative Examples 1 to 5, were added and kneaded for 60 seconds. Then, after 23 parts of hexamethylenetetramine dissolved in 105 parts of water was added thereto and cooled by an air blow, 7 parts of calcium stearate was added. As a result, RCS for shell molding (Samples 1 to 17) produced by using each of the resin compositions of the above Examples and Comparative Examples were obtained.

Production Example 2 of RCS

To a laboratory whirl mixer, 7000 parts of fire-refractory particle (regenerated silica sand) heated to 130 to 140° C., and 105 parts of the above phenolic resin A and 5.25 parts of ethylenebis(stearamide), which constitute Resin composition 18, were each independently added and kneaded for 60 seconds. Then, after 23 parts of hexamethylenetetramine dissolved in 105 parts of water was added thereto and cooled by an air blow, 7 parts of calcium stearate was added. As a result, RCS for shell molding (Sample 18) was obtained.

—Evaluation—

In accordance with the test method described above, each RCS (Samples 1 to 18) obtained as above was subjected to the measurement of flexibility and coefficient of thermal expansion of mold. The results thereof are shown in the following Table 1 and Table 2, together with the production condition of phenolic resin.

TABLE 1 RCS Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Sample 9 Resin composition 1 2 3 4 5 6 7 8 9 Compo- Phenolic resin A A A A A A A B C nents Phenol [%] 80 80 80 80 80 80 80 80 80 1-naphthol [%] 20 20 20 20 20 20 20 20 20 2-naphthol [%] — — — — — — — — — BPA [%] — — — — — — — — — Molar Ratio 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.77 0.50 F/(P + N) Fatty acid amide Ethylenebis Methylenebis Ethylenebis Ethylenebis Stearamide Ethylenebis (stearamide) (stearamide) (behenamide) (erucamide) (stearamide) Amount [part] 5 12 1.5 5 5 5 5 5 5 (based on 100 parts by mass of resin) Mold coefficient of 0.73 0.72 0.75 0.74 0.72 0.72 0.71 0.69 0.77 char- thermal acter- expansion [%] istics flexibility 7.2 10.0 5.8 7.5 7.1 7.4 7.8 5.4 9.8 [mm]

TABLE 2 RCS Sample 10 Sample 11 Sample 12 Sample 13 Sample 14 Sample 15 Sample 16 Sample 17 Sample 18 Resin Composition 10 11 12 13 14 15 16 17 18 Compo- Phenolic resin D E F G A D E F A nents Phenol [%] 90 60 80 20 80 90 60 80 80 1-naphthol [%] 10 40 — — 20 10 40 — 20 2-naphthol [%] — — 20 — — — — 20 — BPA [%] — — — 80 — — — — — Molar Ratio 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 F/(P + N) Fatty acid amide Ethylenebis — Ethylenebis (stearamide) (stearamide) Amount [parts] 5 5 5 5 0 0 0 0 5 (based on 100 parts by mass of resin) Mold coefficient of 0.75 0.71 0.76 0.73 0.76 0.77 0.74 0.78 0.74 char- thermal acter- expansion [%] istics flexibility 6.3 10.5 8.4 3.9 4 2.9 3.4 3.1 7.0 [mm]

As apparent from the results shown in Table 1 and Table 2, every RCS (Samples 1 to 12) obtained by using the resin compositions 1 to 12 which include phenolic resins A to F of Resin production examples 1 to 6 and a predetermined fatty acid amide, which are in accordance with the present invention, has low coefficient of thermal expansion and high flexibility. On the other hand, RCS (Sample 13) obtained by using the phenolic resin G of Resin production example 7, in which phenol and bisphenol A was used as the phenol components, has low flexibility. Further, RCS (Samples 14 to 17) obtained by using the resin compositions 14 to 17, in which no fatty acid amide was added to phenolic resin A, D, E and F, have low flexibility. Furthermore, RCS (Sample 18) obtained by independently adding the phenolic resin A and the fatty acid amide, which constitute a resin composition, into a fire-refractory particle has excellent low coefficient of thermal expansion and excellent flexibility. 

1. A phenolic resin composition for shell molding, comprising as essential components: a phenolic resin obtained by a reaction of a phenol, a naphthol, and an aldehyde; and a fatty acid amide.
 2. The phenolic resin composition for shell molding according to claim 1, wherein a ratio of the phenol to the naphthol is in a range of from 95:5 to 50:50 by mass ratio.
 3. The phenolic resin composition for shell molding according to claim 1, wherein the naphthol comprises at least one of 1-naphthol and 2-naphthol.
 4. The phenolic resin composition for shell molding according to claim 1, wherein a reaction molar ratio among the phenol (P), the naphthol (N), and the aldehyde (F): F/(P+N) is in a range of from 0.40 to 0.80.
 5. The phenolic resin composition for shell molding according to claim 1, wherein the fatty acid amide is present in a range of from 1 to 15 parts by mass based on 100 parts by mass of the phenolic resin.
 6. The phenolic resin composition for shell molding according to claim 1, wherein the fatty acid amide is one of a monamide, a substituted amide, and a bisamide.
 7. The phenolic resin composition for shell molding according to claim 1, wherein the fatty acid amide is a fatty acid bisamide.
 8. The phenolic resin composition for shell molding according to claim 7, wherein the fatty acid bisamide is a saturated fatty acid bisamide.
 9. The phenolic resin composition for shell molding according to claim 1, further comprising a silane coupling agent.
 10. A resin coated sand for shell molding, wherein a fire-refractory particle is coated with the phenolic resin composition for shell molding according to claim
 1. 11. The resin coated sand for shell molding according to claim 10, wherein the phenolic resin composition is present in a range of from 0.2 to 10 parts by mass based on 100 parts by mass of the fire-refractory particle.
 12. A shell mold obtained by fowling and heat-curing the resin coated sand for shell molding according to claim
 10. 13. A process for producing a resin coated sand, comprising the steps of: reacting a phenol, a naphthol, and an aldehyde in the presence of a catalyst to obtain a phenolic resin; and coating a fire-refractory particle with the phenolic resin and a fatty acid amide, which are mixed by melting.
 14. A process for producing a resin coated sand, comprising the steps of: reacing a phenol, a naphthol, and an aldehyde in the presence of a catalyst to obtain a phenolic resin; and coating a fire refractory particle with the phenolic resin and a fatty acid amide, which are used independently.
 15. The process for producing a resin coated sand for shell molding according to claim 13, wherein the catalyst comprises at least one of a divalent metal salt and an oxalic acid.
 16. The process for producing a resin coated sand for shell molding according to claim 14, wherein the catalyst comprises at least one of a divalent metal salt and an oxalic acid. 